An unseasonal heat wave that hit the nation last week would not have happened without human-induced climate change, a group of Japanese scientists said Thursday.

The World Attribution Center (WAC), comprised of independent climate researchers, analyzed the impact of global warming on the nation’s weather between June 16 and June 18. The average temperature 1,500 meters above sea level during this period was 17.2 degrees Celsius, the highest since 1950. Also, the total number of locations where the highest temperature reached 35 degrees C or higher from May 1 through mid-June topped 200, the highest on record.

The heat observed during this period was extremely rare, especially since much of Japan is still in the rainy season, with scorching temperatures coming just a week after the rainy season began in the Kanto-Koshin region, which includes Tokyo.

The extreme temperatures have led to a surge in heatstroke cases. In the week through Sunday, the number of people taken to hospitals by ambulances due to heat-related illnesses shot up to 8,603 nationwide, up from 966 the week before. By prefecture, the number was highest in Tokyo, where 896 people were taken by ambulance, followed by 706 in Aichi and 648 in Saitama.

The heat wave from June 16 to 18 was caused by a unique set of meteorological conditions: a high-pressure system in the Pacific Ocean close to Japan became extremely powerful and a strong atmospheric wave traveled from Europe on the subtropical jet stream, according to the WAC.

The chance of the atmosphere 1,500 meters above sea level hitting 17.2 C under the present climate conditions, where effects of global warming are already manifesting, was 6%, meaning it was a once-in-17-years event, the researchers said.

Meanwhile, based on the historical average, such extreme heat was a phenomenon that would only occur once every 85 years.

If the world had not warmed due to human-induced climate change, this extreme heat could not have occurred regardless of any random weather fluctuations, the researchers concluded.

Japan is set to experience another scorching summer, although according to Weathernews, it may not be as severe as last year’s, which tied with the summer of 2023 as the hottest on record. Temperatures are expected to peak from the end of July through early August, and if the high-pressure systems from the Pacific Ocean and the Tibetan Plateau approach at the same time, the mercury will likely hit 35 C and even 40 C in some places, the private forecaster said.

WAC was founded in May by researchers specializing in a growing field of science called event attribution. The organization’s goal is to publicize the results of its assessment within days of a typhoon, torrential rain or extreme heat — while the impact of the weather event is still fresh in the minds of the public and policymakers.

The researchers started studying the link between weather events and climate change about 15 years ago, but until recently they had only been able to publish their results a few months after extreme weather occurred.